1. Field of the Invention
The present invention relates to a vertical boat used for heat treatment of a semiconductor wafer, and a vertical heat treatment apparatus using the same.
2. Description of the Related Art
In an vertical CVD apparatus for processing a number of semiconductor wafers in a batch manner, wafers are placed in a wafer boat, which is further loaded on the process chamber. The boat is made of a heat-proof quartz or the like, and has a structure having wafer placement positions arranged in the vertical direction such as to have an interval between adjacent wafers when placed.
Published Unexamined Japanese Patent Application (PUJPA) No. 61-247048, and PUJPA No. 3-83730 disclose a structure in which rings each encircling a wafer are placed at the placement positions. With the structure set forth in each of these reference, a wafer is directly set on a ring. A ring serves to improve the uniformity of thickness within the surface of the thin film formed on a wafer.
U.S. patent application 07/765,888 (filed Sep. 26, 1991) discloses a structure in which a number of quartz support stands are provided on each of the rings, and a wafer is set on the stands. With this structure, there is created a gap between the ring and the wafer, and therefore it becomes easy to transfer wafers onto or from a boat, and to supply process gas thereonto.
With the structure set forth in each of the PUJPA Nos. 61-247048 and 3-83730, there must be provided a mechanism for lifting a wafer above from the ring when the wafer is transferred onto or from the boat. Since this lifting mechanism is used in combination with the transfer mechanism, an interval between adjacent wafer placement positions must be taken wide. With the structure disclosed in U.S. application No. 07/765,888, formation process of the support stands is complicated, and therefore it is difficult to achieve a high dimensional precision for each stand. As a result, it is necessary to provide a wide interval between adjacent wafer placement positions so as to compensate for the dimensional precision.
For the reason stated above, the interval between adjacent wafer placement positions is generally set at 9/16 inches in the above-described structures. The interval between adjacent placement positions depends on the number of wafers mounted on a boat having a predetermined length, and must be set as narrow as possible.
Further, with the structures described above, the thickness of the rings cannot be reduced because the rings must have a certain mechanical strength to support a wafer. In particular, with the latter structure, the rings must have a such strength that they can remain during process of the support stands. More specifically, for a semiconductor wafer generally having a thickness of 0.6 mm to 0.7 mm, the thickness of each ring should be 3 mm, about 4.5 to 5 times as thick as the wafer. However, a thick ring is not desirable, because it not only entails a high production cost, but also provides a thermal capacity more than necessary, to the periphery of a wafer.